How can i fill an object that consists of 2D panes into a completely solid (water tight) model?
Looking at the image, the wall seems to have some thickness at the base of the building, but further up inside I can see darker faces indicating planes with no thickness.  I think you will need to manually work through the model to delete any internal faces in edit mode, leaving only external faces. then ensuring all these planes are joined by merging vertexes then finally creating a new face on the bottom to seal it off. 

I am not aware of a simple shortcut.  There is a few hours work needed to convert this into a printable shell.